The subject of this work is the federal system of Spain. In the first general part, the author addresses fundamental questions of federalism. Starting from a definition of terms, the significance of unity and diversity for the existence and development of federal systems is outlined, and their basic structures are elaborated. Additionally, the two fundamental models of federal state organization - the union model and the separation model - are presented, followed by an examination of whether they are suitable for homogeneous or heterogeneous states. In the second part of the work, Spain is classified as a rather heterogeneous state, suggesting that a state organization based on the separation model appears to be the most suitable for Spain, as this model emphasizes the self-determination of its components. The investigation focuses on the existing system in relation to the basic structures of federal systems.